Flexible Multilayer Scattering Substrate Used In OLED

ABSTRACT

A flexible multilayer scattering substrate is disclosed. Built on a flexible supporting layer, the multilayer contains one or more scattering layers and other functional layers so that it can extract the trapped light in substrate and waveguide of an OLED. The processing of each layer is fully compatible with large area, flexible OLED manufactory, and by controlling processing conditions of each incorporated layer, the substrate microstructure can be tuned. Topographic features can be created on the top surface of substrate by changing the thickness and properties of the multilayer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/200,596, filed Jul. 1, 2016, which is a non-provisional of U.S.Patent Application Ser. No. 62/189,961, filed Jul. 8, 2015, both ofwhich are incorporated herein by reference in their entireties.

PARTIES TO A JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Regents of the University of Michigan,Princeton University, University of Southern California, and theUniversal Display Corporation. The agreement was in effect on and beforethe date the claimed invention was made, and the claimed invention wasmade as a result of activities undertaken within the scope of theagreement.

FIELD OF THE INVENTION

The present invention relates to flexible multilayer scatteringsubstrates, and devices, such as organic light emitting diodes and otherdevices, including the same.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the following structure:

In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

A ligand may be referred to as “photoactive” when it is believed thatthe ligand directly contributes to the photoactive properties of anemissive material. A ligand may be referred to as “ancillary” when it isbelieved that the ligand does not contribute to the photoactiveproperties of an emissive material, although an ancillary ligand mayalter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled inthe art, a first work function is “greater than” or “higher than” asecond work function if the first work function has a higher absolutevalue. Because work functions are generally measured as negative numbersrelative to vacuum level, this means that a “higher” work function ismore negative. On a conventional energy level diagram, with the vacuumlevel at the top, a “higher” work function is illustrated as furtheraway from the vacuum level in the downward direction. Thus, thedefinitions of HOMO and LUMO energy levels follow a different conventionthan work functions.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

Due to the index mismatching at the interfaces of different index layersof an OLED, internal reflections occur, resulting in large part of lightbeing trapped inside of the device. As the result, only 20% of generatedlight is outcoupled without any extracting method in a bottom emittingOLED. By incorporating a scattering layer, the emission angle is shiftedand the internal reflection is reduced. The method belongs to the socalled internal extraction structure (IES). One of the most importantrequirements of the IES is compatibility with lateral OLED fabrication.Specifically, the roughness of the top surface which contacts the OLED.For example, Hong-Wei Chang et al., demonstrated that by milling thescattering particles for 24 hours, fine top surface can be achievedwithout a further planarizing layer (Journal of Applied Physics 113,204502, 2013). However, milling and the following filtering processesare not cost efficient. Most of the time, scattering alone cannotprovide such fine roughness requirement, and a planarizing layer on topcan flatten the surface. Regarding flexible OLED, another requirement isthat the final substrate has to be a stand-alone film. Thus thescattering layer must not cause internal stress or crack under externalstress. Due to the internal stress caused by the nanoparticles blendedwith polymer, the scattering layer alone usually fails to meet thesecriteria.

U.S. Pat. No. 7,579,054 discloses a flexible substrate comprising aresin composition layer containing an inorganic layered compound and aresin matrix. While it is a good example of the concept using aninorganic compound blended into a polymer matrix as a scattering layerand applied it to a flexible substrate, in terms of flexibleapplications, by using the scattering layer alone as the substrate, itis usually not satisfactory for OLED fabrication, even though theconcept has been well studied and successfully applied on rigidsubstrates. On the other hand, U.S. Patent Application No. 20140264316discloses an outcoupling system containing one or more scattering layerson top of a rigid substrate for OLED application. There is therefore acontinuing need in the art for OLED multilayer substrates, and inparticular flexible multilayer substrates for the next generationflexible display and light applications, wherein the flexible substrateis needed as the first supporting layer. There is also a continuing needin the art for addressing issues such as surface roughness, stress andprocessing conditions. The current invention fulfills these needs, byapplying more functional layers with the same fabrication process,ensuring not only substrate flexibility, but bringing as well additionaloutcoupling features to the substrate.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a flexible, multi-layersubstrate for an organic light emitting diode (OLED), comprising: aflexible support layer having a thickness of less than 50 μm; a lightscattering layer disposed over the support layer, wherein the lightscattering layer comprises light scattering particles dispersed in apolymer matrix; and a planarization layer disposed over the lightscattering layer. In one embodiment, the flexible support layer has athickness of less than 10 μm. In another embodiment, the flexiblesupport layer has a thickness of less than 5 μm. In one embodiment, thesupport layer is doped with a portion of the light scattering particlesfrom the light scattering layer at an interface between the supportlayer and the light scattering layer. In another embodiment, the supportlayer is substantially free of light scattering particles from the lightscattering layer at an interface between the support layer and the lightscattering layer. In one embodiment, the support layer comprises atleast one of PMMA, polyamide and PDMS. In another embodiment, the lightscattering layer comprises a substantially uniform blend of scatteringparticles and polymer matrix. In one embodiment, the light scatteringparticle size is between 100 nm and 600 nm. In one embodiment, thescattering particles and polymer matrix are index mismatched. In oneembodiment, the scattering particles comprise at least one of titaniumoxide, zirconium oxide and zinc oxide. In one embodiment, the polymermatrix of the light scattering layer and the support layer are composedof the same polymer. In one embodiment, the loading ratio of thescattering particles is between 5% and 50%. In one embodiment, thethickness of the light scattering layer is between 0.1 and 10 μm. Inanother embodiment, the planarization layer comprises a plurality oflayers. In one embodiment, the planarization layer and the support layercomprise the same material. In one embodiment, the planarization layercomprises at least one of silicon oxide, aluminum oxide, siliconnitride, indium zinc oxide, indium tin oxide and zinc aluminum oxide. Inone embodiment, scattering centers of the light scattering layer havevariable distributions over a normal direction of the flexiblesubstrate. In one embodiment, the light scattering layer is positionedalong a neutral plane of the multi-layer substrate. In anotherembodiment, normal directional transmission of the substrates is between10% and 60%. In one embodiment, the flexible substrate has a refractiveindex greater than 1.5. In one embodiment, the top surface of theplanarization layer includes a topographic structure having a thicknessof between 10 nm and 2 μm. In another embodiment, the thickness of thetopographic structure is between 10 nm and 200 nm.

In another aspect, the invention relates to an OLED device, comprisingat least one OLED stack disposed over the planarization layer of aflexible substrate of the invention.

In another aspect, the invention relates to a method of fabricating aflexible substrate for an organic light emitting diode (OLED),comprising: solution processing a support layer having a thickness ofless than 50 μm on a rigid plate; curing the support layer; disposing alight scattering layer comprising light scattering particles dispersedin a polymer matrix on the support layer; curing the light scatteringlayer; disposing a planarization layer on top of the light scatteringlayer; and curing the planarization layer. In one embodiment, the methodfurther comprises: curing the flexible substrate. In one embodiment,curing comprises soft baking. In another embodiment, curing comprisesroom temperature annealing. In one embodiment, curing the support layerand curing the light scattering layer creates a support layersubstantially free of doped light scattering particles from the lightscattering layer at an interface between the support layer and the lightscattering layer. In another embodiment, curing the support layer andcuring the light scattering layer creates a support layer doped with aportion of the light scattering particles from the light scatteringlayer at an interface between the support layer and the light scatteringlayer. In one embodiment, the rigid plate comprises at least one ofglass, a silicon wafer, a metal plate, a sapphire plate and a plasticplate. In another embodiment, curing the flexible substrate compriseshigh temperature curing at over 150° C. for at least 1 hour. In oneembodiment, all layers are processed with at least one of doctor bladecoating and slot die coating. In one embodiment, the planarization layeris processed by at least one of plasma-enhanced chemical vapordeposition (PECVD), physical vapor deposition (PVD), and a vacuummethod. In one embodiment, the method further comprises: peeling theflexible substrate off of the rigid plate.

In another aspect, the invention relates to a method of fabricating anOLED comprising a method of the invention for fabricating a flexiblesubstrate of the invention further comprising: fabricating an OLED ontop of the flexible substrate. In one embodiment, the method furthercomprises: fabricating an OLED on top of the flexible substrate; andpeeling the flexible substrate off of the rigid plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 is a schematic showing the elements of a flexible substrate,including a flexible support layer, a light scattering layer, and aplanarization layer, and the resulting different microstructures of thefinal flexible substrate depending on different processing conditions.

FIG. 4 shows a scanning electron microscopy (SEM) cross section image ofa stand-alone flexible substrate containing a soft baked supportinglayer and scattering layer, where a clean interface is observed betweenthe layers.

FIG. 5 shows an SEM cross section image of a stand-alone flexiblesubstrate containing a room temperature baked supporting layer andscattering layer on top, where a well-mixed diffusion zone interface isobserved between the layers, instead of a clean interface.

FIG. 6 is a chart showing the transmittance of flexible scatteringsubstrates with different loading ratios.

FIG. 7 is a chart showing the relation between index change vs. theoptical modes distribution of a bottom emission OLED (simulation).

FIG. 8 is a chart showing current-voltage-luminance plots.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a more clear comprehension of the present invention, whileeliminating, for the purpose of clarity, many other elements found insystems and methods relating to scattering substrates and theirapplication to OLED. Those of ordinary skill in the art may recognizethat other elements and/or steps are desirable and/or required inimplementing the present invention. However, because such elements andsteps are well known in the art, and because they do not facilitate abetter understanding of the present invention, a discussion of suchelements and steps is not provided herein. The disclosure herein isdirected to all such variations and modifications to such elements andmethods known to those skilled in the art.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are described.

As used herein, each of the following terms has the meaning associatedwith it in this section.

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e., to at least one) of the grammatical object of thearticle. By way of example, “an element” means one element or more thanone element.

“About” as used herein when referring to a measurable value such as anamount, a temporal duration, and the like, is meant to encompassvariations of ±20%, ±10%, ±5%, ±1%, and ±0.1% from the specified value,as such variations are appropriate.

Ranges: throughout this disclosure, various aspects of the invention canbe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Where appropriate, the description of a range should beconsidered to have specifically disclosed all the possible subranges aswell as individual numerical values within that range. For example,description of a range such as from 1 to 6 should be considered to havespecifically disclosed subranges such as from 1 to 3, from 1 to 4, from1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well asindividual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5,5.3, and 6. This applies regardless of the breadth of the range.

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence is described in moredetail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporatedby reference.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, a cathode 160, and a barrier layer 170.Cathode 160 is a compound cathode having a first conductive layer 162and a second conductive layer 164. Device 100 may be fabricated bydepositing the layers described, in order. The properties and functionsof these various layers, as well as example materials, are described inmore detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which areincorporated by reference.

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is m-MTDATA dopedwith F₄-TCNQ at a molar ratio of 50:1, as disclosed in U.S. PatentApplication Publication No. 2003/0230980, which is incorporated byreference in its entirety. Examples of emissive and host materials aredisclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which isincorporated by reference in its entirety. An example of an n-dopedelectron transport layer is BPhen doped with Li at a molar ratio of 1:1,as disclosed in U.S. Patent Application Publication No. 2003/0230980,which is incorporated by reference in its entirety. U.S. Pat. Nos.5,703,436 and 5,707,745, which are incorporated by reference in theirentireties, disclose examples of cathodes including compound cathodeshaving a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thetheory and use of blocking layers is described in more detail in U.S.Pat. No. 6,097,147 and U.S. Patent Application Publication No.2003/0230980, which are incorporated by reference in their entireties.Examples of injection layers are provided in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety. A description of protective layers may be found in U.S. PatentApplication Publication No. 2004/0174116, which is incorporated byreference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove outcoupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. Pat. No. 7,431,968, which is incorporated by reference in itsentirety. Other suitable deposition methods include spin coating andother solution based processes. Solution based processes are preferablycarried out in nitrogen or an inert atmosphere. For the other layers,preferred methods include thermal evaporation. Preferred patterningmethods include deposition through a mask, cold welding such asdescribed in U.S. Pat. Nos. 6,294,398 and 6,468,819, which areincorporated by reference in their entireties, and patterning associatedwith some of the deposition methods such as ink-jet and OVJD. Othermethods may also be used. The materials to be deposited may be modifiedto make them compatible with a particular deposition method. Forexample, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the presentinvention may further optionally comprise a barrier layer. One purposeof the barrier layer is to protect the electrodes and organic layersfrom damaging exposure to harmful species in the environment includingmoisture, vapor and/or gases, etc. The barrier layer may be depositedover, under or next to a substrate, an electrode, or over any otherparts of a device including an edge. The barrier layer may comprise asingle layer, or multiple layers. The barrier layer may be formed byvarious known chemical vapor deposition techniques and may includecompositions having a single phase as well as compositions havingmultiple phases. Any suitable material or combination of materials maybe used for the barrier layer. The barrier layer may incorporate aninorganic or an organic compound or both. The preferred barrier layercomprises a mixture of a polymeric material and a non-polymeric materialas described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporatedby reference in their entireties. To be considered a “mixture”, theaforesaid polymeric and non-polymeric materials comprising the barrierlayer should be deposited under the same reaction conditions and/or atthe same time. The weight ratio of polymeric to non-polymeric materialmay be in the range of 95:5 to 5:95. The polymeric material and thenon-polymeric material may be created from the same precursor material.In one example, the mixture of a polymeric material and a non-polymericmaterial consists essentially of polymeric silicon and inorganicsilicon.

Devices fabricated in accordance with embodiments of the invention canbe incorporated into a wide variety of electronic component modules (orunits) that can be incorporated into a variety of electronic products orintermediate components. Examples of such electronic products orintermediate components include display screens, lighting devices suchas discrete light source devices or lighting panels, etc. that can beutilized by the end-user product manufacturers. Such electroniccomponent modules can optionally include the driving electronics and/orpower source(s). Devices fabricated in accordance with embodiments ofthe invention can be incorporated into a wide variety of consumerproducts that have one or more of the electronic component modules (orunits) incorporated therein. Such consumer products would include anykind of products that include one or more light source(s) and/or one ormore of some type of visual displays. Some examples of such consumerproducts include flat panel displays, computer monitors, medicalmonitors, televisions, billboards, lights for interior or exteriorillumination and/or signaling, heads-up displays, fully or partiallytransparent displays, flexible displays, laser printers, telephones,cell phones, tablets, phablets, personal digital assistants (PDAs),laptop computers, digital cameras, camcorders, viewfinders,micro-displays, 3-D displays, vehicles, a large area wall, theater orstadium screen, or a sign. Various control mechanisms may be used tocontrol devices fabricated in accordance with the present invention,including passive matrix and active matrix. Many of the devices areintended for use in a temperature range comfortable to humans, such as18° C. to 30° C., and more preferably at room temperature, i.e., 20-25°C., but could be used outside this temperature range, for example, from−40° C. to +80° C.

In one aspect, the invention relates to a flexible multilayer scatteringsubstrate for an organic light emitting diode (OLED). Referring to FIG.3, in some embodiments 340 and 360, the flexible multilayer scatteringsubstrate is a single unit containing at least three sub-layers: thefirst sub-layer 310 is a flexible supporting sub-layer having athickness of less than 50 μm, the second sub-layer 320 disposed over thesupport sub-layer, is a light scattering sub-layer comprising lightscattering particles dispersed in a polymer matrix, and the thirdsub-layer 330 is a planarization sub-layer disposed over the lightscattering sub-layer. By sandwiching the light scattering sub-layer 320between the flexible supporting sub-layer 310 and the planarizationsub-layer 330, the scattering sub-layer 320 is therefore located at, orclose to, the stress neutral plane, which helps improving theflexibility of the flexible multilayer scattering substrate.

Referring to FIGS. 3 and 4, owing to processing conditions describedelsewhere herein including soft baking curing, in one embodiment 340,the flexible support sub-layer 310 is substantially free of lightscattering particles from the light scattering sub-layer 320, at aninterface 350 between the flexible support sub-layer 310 and the lightscattering sub-layer 320.

Referring to FIGS. 3 and 5, owing to processing conditions describedelsewhere herein including room temperature annealing, in one embodiment360, the flexible support sub-layer 310 is doped with a portion of thelight scattering particles from the light scattering sub-layer 320 at adiffusion zone interface 370 between the flexible support sub-layer 310and the light scattering sub-layer 320. A diffusion zone interfaceresults in advantages such as lower internal stress of the flexiblemultilayer scattering substrate.

In some embodiments, the materials used in the first sub-layer 310 canbe polymers such as PMMA, polyimide, or PDMS. In one embodiment, thethickness of the flexible support sub-layer is less than 2 In otherembodiments, the thickness is anywhere between 2 μm and Compared withother flexible substrates such as PEN, PET, or metal foil, thethicknesses of which are usually well over 50 the thickness advantageensures the flexibility of the final product. including the flexiblemultilayer scattering substrate or an OLED.

In some embodiments, the scattering sub-layer consists of blendedscattering particles in a polymer matrix. In one embodiment, thescattering sub-layer requires a refractive index mismatch between thescattering particles and the polymer matrix. In another embodiment, thelarger this mismatch is, the stronger the scattering will be. In oneembodiment, the scattering particle size is between 100 to 600 nm. Inanother embodiment, the scattering particle material can be titaniumoxide, zirconium oxide, or zinc oxide. In another embodiment, the matrixis the polymer used in the first supporting sub-layer. In anotherembodiment, the loading ratio (weight percentage of scattering particlesover all solids) is between 5% and 50%. In another embodiment, thethickness of the scattering sub-layer is between 1 to 5 In anotherembodiment, the thickness of the scattering sub-layer is between 0.1 to10 In another embodiment, the scattering centers have differentdistributions over the direction normal to the flexible multilayerscattering substrate. In another embodiment, by incorporating ascattering sub-layer, the normal directional light transmission of theflexible multilayer scattering substrate is between 10% and 60%.

In one embodiment, poly(methyl methacrylate) (PMMA) is used as polymermatrix of the scattering sub-layer, and 300 nm titanium oxide (TiOx)nanoparticles (NPs) as scattering particles. PMMA has a refractive indexof 1.5 and TiOx has a refractive index of 2.2. In one embodiment, 1 gPMMA was dissolved into 10 ml dimethylacetamide (DMAc) as solution A. Asecond solution B was prepared, containing 0.45 g TiOx nanoparticlessuspended into 9.5 ml DMAc, with 0.5 ml diethylene glycol monobutylether as surfactant. Solutions A and B were sonicated for 6 hours beforethey are mixed, another 6 hour sonication was performed, and theresulting composition was used to fabricate a scattering sub-layer. Theresulting thin film scattering sub-layer has a loading ratio of 30%.

Beside the refractive index difference, the scattering effect can betuned also by varying the particle size, the loading ratio of particlesover the total solid weight, and the thickness of the scattering layer.In some embodiments, the particle size is better close to scatteringwavelength. Too small size cannot give any scattering, while too largesize will affect the film quality in terms of internal stress andsmoothness. In some embodiments, a suitable range is between 100 nm to600 nm. For a given scattering layer thickness, the higher the NPsloading ratio, the more scattering is observed, therefore lowering thetransmittance of the film. FIG. 6 shows the transmittance of a flexiblemultilayer scattering substrate of 7.5 μm total thickness, including a1.25 μm thick scattering sub-layer sandwiched between the flexiblesupporting sub-layer and the planarization sub-layer, at variousscattering particles loading ratios. The 300 nm NPs (30% loading ratio)afford a good scattering spectrum, where a relative flat transmissioncovers the visible range. For 550 nm incident light, a 32% transmittanceis observed. FIG. 6 also shows the transmission of other loading ratiowith 300 nm TiOx in PMMA. Too high loading ratio is not desired becauseof the resulting high internal stress caused by NPs' aggregation. Insome embodiments, a loading ratio range from 5% to 50% is generally goodfor scattering and film quality. The consideration of scatteringsub-layer thickness lays on the overall stress and flexibility of theflexible multilayer scattering substrate. A thick scattering sub-layerresults in large stress mismatch in the whole stack.

In some embodiments, the planarization sub-layer contains one or moresub-layers. In another embodiment, the material used for theplanarization sub-layer is the same as the first flexible supportingsub-layer, or can be an inorganic material such as silicon oxide,aluminum oxide, silicon nitride, indium zinc oxide, indium tin oxide, orzinc aluminum oxide. In another embodiment, the index of the substratecontacting OLED is larger than 1.5.

In some embodiments, a topographic structure can be created on top ofthe flexible multilayer scattering substrate, depending on the thicknessof the planarization sub-layer and the composition of the scatteringsub-layer. In one embodiment, the final topographic structure hasfeature size between 100 nm to 2 μm. In another embodiment, the finaltopographic structure has feature peak to valley between 10 nm to 200nm.

In some embodiments, the matrix polymer used is an optically clearmaterial. The index of the matrix affects the overall outcouplingenhancement. FIG. 7 shows a simulation of light distribution versus theindex of the substrate. The outcoupled (I_OC) mode indicates lightemitted out of the device without any extracting method. Under thiscondition, only 25% of generated light is in I_OC mode. Total internalreflection occurs at the interface of substrate and air, holding 20% ofthe light in the substrate (I_SG mode). Another 20% of that light istrapped in the waveguided (I_GM) mode, due to the higher index of ITOand organic materials than that of the substrate. About 30% of the lightis eventually evanescent (I_EC_mode) through the surface plasmon loss atthe interface of the organic layer and the metal electrode. Anothersmall part of light is absorbed by the organic material and metal(I_AL). With the index of substrate approaching to ITO/organic, more andmore light is converted into I_SG mode. The light in I_SG mode is thenpotentially outcoupled by scattering.

In some embodiments, the surface of the flexible multilayer scatteringsubstrate can be tuned by varying the thickness of the planarizationsub-layer. A typical surface of the scattering sub-layer is too rough tosupport OLED growth. Table 1 shows the root mean square (R.M.S) surfaceroughness of a scattering sub-layer including 300 nm NPs blended withpolymer as measured by atomic-force microscopy (AFM). With the largestpeak to valley at over 200 nm, the R.M.S. is 58.6 nm. With an increasingin the thicknesses of the planarization layer, the roughness of thesurface decreases. With the thickness up to 5 the surface topographicfeature disappears, showing a smooth surface with 8.4 nm roughness.Therefore various surface morphologies can be obtained by changing thethickness of the planarization sub-layer. For example, a peak to valleyroughness of 100 nm is obtained with the planarization layer thicknessof 2 In some embodiments, under this condition, an OLED can befabricated on top of this rough surface so that the topographic featurecan be utilized for outcoupling.

TABLE 1 roughness of flexible substrate with planarization layerPlanarization Thickness (μm) 0 1.0 2.2 4.7 R.M.S. Roughness (nm) 58.612.8 14.3 8.4

In one aspect, the invention relates to an OLED. In some embodiments,the OLED is fabricated on top of a flexible multilayer scatteringsubstrate of the invention. In one embodiment, the OLED is a bottomemission OLED. In another embodiment, the OLED is a top emission OLED.In one embodiment, an OLED was fabricated on the flexible multilayerscattering substrate. The substrate has a total thickness of 7.5 μm witha 1.25 μm thick scattering sub-layer sandwiched between the flexiblesupporting sub-layer and the planarization sub-layer. The substrateshows a 32% transmittance at 550 nm incident light, and a roughness of2.3 nm of the top surface. An OLED without the scattering sub-layer wasalso fabricated for comparison purposes. The reference substrate onlyhas the supporting sub-layer and planarization sub-layer. Thetransmittance at 550 nm incident light is 86%, and the roughness is 0.3nm. Due to the smooth surface afforded by the planarization sub-layer,the two devices give nearly the same current-voltage curve as shown inFIG. 8. In terms of efficiency, the reference device has a currentefficiency of 44 cd/A at 1000 nits. By including a scattering sub-layer,the efficiency increased to 67 cd/A. Due to the broad scattering overthe visible range, there is no significant derivation of the scatteringdevice emission spectrum compared to the reference device.

In one aspect, the invention relates to a process of fabricating aflexible multilayer scattering substrate. In one embodiment, the processstarts with solution processing the first supporting sub-layer on arigid plate. In one embodiment, the rigid plate could be glass, siliconwafer, metal plate, sapphire plate, plastic plate. In anotherembodiment, the resulting interface between each sub-layer depends onthe annealing or curing process after deposition. In another embodiment,a soft baking quickly cures the preceding film and creates a clearinterface with the subsequent film. In another embodiment, a slow curingprocess takes place under room temperature annealing, which results inblurring of interface between sub-layers, and doping of the underneathpreceding sub-layer. In another embodiment, the final step for theentire flexible multilayer scattering substrate is a high temperaturecuring at 150° C. over 1 hour. In another embodiment, all sub-layers canbe processed by doctor blade coating, slow die coating, and any otherlarge area coating methods of solution processing known in the art. Inanother embodiment, the planarization sub-layer(s) can also be done byPECVD, PVD, and any other vacuum coating methods known in the art. Inanother embodiment, the process of fabricating flexible multilayerscattering substrates of the invention ends up with peeling off thewhole flexible film from the rigid plate.

In some embodiments, a process of the invention starts with coating thefirst supporting flexible sub-layer on a rigid plate, and ends withpeeling the whole flexible multilayer scattering substrate off the rigidplate. The flexible supporting sub-layer serves the purpose of flexiblebase for the following scattering sub-layer. It also provides a goodcontact surface with the underneath rigid plate so that a large areauniform coating can be achieved. The materials of this sub-layer areplastic which can be cast from liquid state, such as PMMA, polyimide,PDMS, or any type of resin known in the art. Then a scattering sub-layeris coated as an outcoupling sub-layer. In some embodiments, thescattering sub-layer covers the full area of the supporting sub-layer.In other embodiments, one or more planarization sub-layers arethereafter deposited. In some embodiments, after deposition andtreatments of all sub-layers, the whole flexible multilayer substratestack goes through final annealing. In one embodiment, final annealingis performed at, or at more than, 150° C. In another embodiment, finalannealing is performed for, or for more than, an hour. In otherembodiments, the temperature used for final annealing is higher than thecrosslink or thermal set temperature of the matrix materials. In otherembodiments, the final step is to release the flexible multilayersubstrate from the rigid plate.

In one embodiment, the annealing of each sub-layer affects themicrostructure of the flexible multilayer substrate. In one embodiment,the annealing affects the scattering center density distribution overthe normal direction of the film. In one embodiment, soft baking, e.g.,at 90° C. for 3 min, is applied to each sub-layer after coating, asshown for example in FIG. 3 (top arrow process). Soft baking quicklydrives out all solvent and solidifies the film, and as a result, themicrostructure of each sub-layer is substantially preserved. Referringnow to FIG. 4, the SEM cross section image of a stand-alone flexiblemultilayer film containing soft baked supporting sub-layer andscattering sub-layer, reveals a clear interface 350. The scatteringcenters are confined in the scattering sub-layer due to the fast dryingprocess.

In other embodiments, each sub-layer goes under room temperatureannealing after deposition, as shown for example in FIG. 3 (bottom arrowprocess). The curing time is longer than the soft baking. In someembodiments, the curing time is about 5 to 10 min longer, depending onthe thickness of the sub-layer. During the slow solvent evaporationprocess, there is therefore time for the two sub-layers to penetrate ordiffuse into each other. Referring now to FIG. 5, the SEM cross sectionimage of a stand-alone flexible multilayer film containing roomtemperature baked supporting sub-layer and scattering sub-layer on top,no clear interface can be observed, but rather a diffusion zone 370 canbe observed. When the scattering center blending sub-layer penetratesinto other sub-layers, a self-doping takes place. As readily apparent,particles located in the supporting sub-layer illustrate the result ofthe doping process. The penetration depth and doping concentration aredepending on the annealing process and the materials of the solvent,matrix, and blending scattering particles. The self-doping processadvantageously reduces internal stress.

The disclosures of each and every patent, patent application, andpublication cited herein are hereby incorporated herein by reference intheir entirety. While this invention has been disclosed with referenceto specific embodiments, it is apparent that other embodiments andvariations of this invention may be devised by others skilled in the artwithout departing from the true spirit and scope of the invention.

1. A flexible, multi-layer substrate for an organic light emitting diode(OLED), comprising: a flexible support layer having a thickness of lessthan 50 μm; a light scattering layer disposed over the support layer,wherein the light scattering layer comprises light scattering particlesdispersed in a polymer matrix; and a planarization layer disposed overthe light scattering layer. 2-4. (canceled)
 5. The flexible substrate ofclaim 1, wherein the support layer is substantially free of lightscattering particles from the light scattering layer at an interfacebetween the support layer and the light scattering layer.
 6. Theflexible substrate of claim 1, wherein the support layer comprises atleast one of PMMA, polyimide and PDMS.
 7. (canceled)
 8. The flexiblesubstrate of claim 1, wherein the light scattering particle size isbetween 100 nm and 600 nm.
 9. (canceled)
 10. The flexible substrate ofclaim 1, wherein the scattering particles comprise at least one oftitanium oxide, zirconium oxide and zinc oxide. 11-12. (canceled) 13.The flexible substrate of claim 1, wherein the thickness of the lightscattering layer is between 0.1 and 10 μm.
 14. The flexible substrate ofclaim 1, wherein the planarization layer comprises a plurality oflayers. 15-17. (canceled)
 18. The flexible substrate of claim 1, whereinthe light scattering layer is positioned along a neutral plane of themulti-layer substrate. 19-20. (canceled)
 21. The flexible substrate ofclaim 1, wherein the top surface of the planarization layer includes atopographic structure having a thickness of between 10 nm and 2 μm. 22.The flexible substrate of claim 21, wherein the thickness of thetopographic structure is between 10 nm and 200 nm.
 23. (canceled)
 24. Amethod of fabricating a flexible substrate for an organic light emittingdiode (OLED), comprising: solution processing a support layer having athickness of less than 50 μm on a rigid plate; curing the support layer;disposing a light scattering layer comprising light scattering particlesdispersed in a polymer matrix on the support layer; curing the lightscattering layer; disposing a planarization layer on top of the lightscattering layer; and curing the planarization layer.
 25. The method ofclaim 24, further comprising: curing the flexible substrate.
 26. Themethod of claim 24, wherein curing comprises soft baking.
 27. The methodof claim 24, wherein curing comprises room temperature annealing. 28.The method of claim 24, wherein curing the support layer and curing thelight scattering layer creates a support layer substantially free ofdoped light scattering particles from the light scattering layer at aninterface between the support layer and the light scattering layer. 29.The method of claim 24, wherein curing the support layer and curing thelight scattering layer creates a support layer doped with a portion ofthe light scattering particles from the light scattering layer at aninterface between the support layer and the light scattering layer. 30.(canceled)
 31. The method of claim 24, wherein curing the flexiblesubstrate comprises high temperature curing at over 150° C. for at least1 hour.
 32. The method of claim 24, wherein all layers are processedwith at least one of doctor blade coating and slot die coating.
 33. Themethod of claim 24, wherein the planarization layer is processed by atleast one of plasma-enhanced chemical vapor deposition (PECVD), physicalvapor deposition (PVD), and a vacuum method.
 34. (canceled)
 35. A methodof fabricating an OLED comprising the method of claim 24 furthercomprising: fabricating an OLED on top of the flexible substrate. 36.(canceled)